1. Field of the Invention
The present invention relates generally to a sheet processing apparatus, an image forming system, and a sheet conveying method, and more particularly, to a sheet processing apparatus that folds a sheet of recording medium (hereinafter, “sheet”) such as plain paper, transfer paper, printing paper, or an overhead transparency film conveyed to the apparatus, an image forming system including the sheet processing apparatus and an image forming apparatus such as a copier, a printer, facsimile, or a digital multifunction peripheral, and a sheet conveying method performed by the sheet processing apparatus.
2. Description of the Related Art
A technique of folding a sheet by deflecting a sheet in a space between two pairs of rollers and pinching the deflecting portion in a nip formed between another pair of rollers is already known. Known examples of such a technique include that disclosed in Japanese Laid-open Patent Publication No. 2007-277006.
The technique disclosed in Japanese Laid-open Patent Publication No. 2007-277006 provides a method for folding a medium by a folding apparatus which includes a rotatable folding cylinder, a first rotatable press member capable of engaging with the folding cylinder to form a first folding pinch, a second rotatable press member capable of engaging with the folding cylinder to form a second folding pinch, and medium feed means. The method includes: a) feeding, by the medium feed means, a medium toward the cylinder located midway between the first pinch and the second pinch; b) directing the medium into the first pinch by rotating the cylinder in a first direction; c) forming a slack in the medium at a position between the feed means and the cylinder; and d) conveying the slack of the medium into the second pinch by rotating the cylinder in a second direction, which is opposite to the first direction.
The conventional technique described above folds a sheet by causing one of the two pairs of cylinders (hereinafter, referred to as “two pairs of rollers”) to convey the sheet forward while causing the other one to convey the sheet backward so that the sheet is deflected at the position between the two pairs of rollers, and pinching the deflected portion in a roller nip.
Such a sheet folding apparatus that folds a sheet by deflecting the sheet in a space between two pairs of conveying members and pinching the deflected portion in a nip of another pair of rollers generally has a path for conveying the sheet to a downstream apparatus and a path for performing the folding process separately. This is because the folding process requires a space for deflecting the sheet by rotating the conveying members backward. Furthermore, to fold a sheet in half-fold, it is necessary to guide a leading end of the sheet to another path than the path for conveying the sheet. Accordingly, conventionally, apparatuses capable of a plurality of folding types have been disadvantageously large in size due to the necessity of having the plurality of paths and space.
Meanwhile, a sheet folding apparatus in which a path for conveying a sheet to a downstream apparatus and a path for performing a folding process are not separated but a sheet folded on a conveying path is conveyed to a downstream apparatus along the same conveying path, is already known. An example of such a sheet folding apparatus is disclosed in Japanese Patent No. 3257899.
This sheet folding apparatus includes: first and third conveying means which convey a sheet substantially horizontally; second conveying means which conveys the sheet conveyed by the first conveying means substantially vertically and is to be driven forward and backward; and switching means which switches from one sheet conveying path to another in a region surrounded by the first, second, and third conveying means. A length of the sheet conveying path between the first and third conveying means is set so as to satisfy a predetermined relationship. Reversing means, which turns a sheet upside down by changing timing at which the switching means should switch the sheet conveying path, also serves as folding means which folds the sheet at a predetermined position.
This sheet folding apparatus includes a member referred to as a flapper for switching a path of a sheet leading end and for assisting a folding process at a bifurcating point. The flapper serves not only as the switching means but also as the folding means.
The flapper of the sheet folding apparatus disclosed in Japanese Patent No. 3257899 includes an upper bifurcating claw and a lower bifurcating claw. The flapper is configured such that the lower bifurcating claw rotates so as to follow rotation of the upper bifurcating claw at the sheet conveying path surrounded by the first through third pairs of conveying rollers. By rotating the lower bifurcating claw in the manner to follow the rotation of the upper bifurcating claw or, in other words, by rotating the upper bifurcating claw and the lower bifurcating claw in synchronization with each other, the reversing means can function also as the folding means.
However, this configuration in which the upper bifurcating claw and the lower bifurcating claw are rotated in synchronization with each other has the following disadvantage. A crease is formed by making use of a folding edge of the lower bifurcating claw. The conveying members are rotated backward by an amount which depends on a position where the crease is to be formed. In contrast to the technique disclosed in Japanese Laid-open Patent Publication No. 2007-277006, with this configuration, it is impossible to fold a sheet by guiding a deflected portion, which is formed by rotating the conveying members backward, to a nip between the conveying members which perform folding.
For this reason, to fold a sheet using the technique disclosed in Japanese Patent No. 3257899 by guiding a deflected portion, which is formed by rotating the conveying members backward, to the nip between the conveying members which perform folding, it is necessary to add means therefor. However, addition of such means will undesirably result in an increase in size of the apparatus.
Under the circumstances, there is a need for downsizing a sheet processing apparatus capable of folding a sheet using rollers.